When a continuous casting slab is rolled to manufacture a heavy gauge steel plate, it is important to reduce centerline segregation and the central porosity volume: such centerline segregation and central porosity form in the central part of the continuous casting slab in the thickness direction. Thus, reduction is carried out on the slab inside and outside a continuous casting machine in order to improve the internal quality of the slab. A heavy gauge steel plate that is obtained by rolling a slab of excellent internal quality as a material is of excellent internal quality. Formation of internal defects caused by central porosity is checked in such a heavy gauge steel plate.
The following are conventional arts of reduction on a slab: Japanese Patent No. 1480540 (Patent Literature 1) discloses the art of carrying out heavy reduction on the central part and both side parts of a slab in order with convex rolls and flat rolls arranged more downstream than a slab cutting machine of a continuous casting machine.
In the art described in Patent Literature 1, the efficiency of the reduction is decreasing as surface temperature of the slab is falling because the reduction is carried out outside the continuous casting machine. Thus, it is necessary to maintain reduction force and to heavily invest in plant and equipment.
Japanese Patent No. 4296985 (Patent Literature 2) discloses the art of rolling-reducing the center part of a cast slab in width by 3 to 15 mm when the solid-phase ratio at the center of the cast slab in the thickness direction is 0.80 or more after bulging the cast slab by 2 to 20 mm in a continuous caster. Japanese Patent No. 4813817 (Patent Literature 3) discloses the art of specifying temperature at the center part of a slab when rolling-reduction is started at a completely solidified place of the slab and temperature on the slab surface when the rolling-reduction is ended upon the rolling-reduction on the slab in continuous casting, and making the rolling-reduction quantity X when difference between the surface temperature at the end of the rolling-reduction and the temperature at the center at the start of the rolling-reduction is 600° C., to be a predetermined quantity or more. Japanese Laid-open Patent Publication No. 2007-296542 (Patent Literature 4) discloses the art of specifying the relation between the rolling-reduction amount and the center porosity volume of a cast slab when the slab at the end of solidification is rolling-reduced as a whole using one pair of upper and lower rolling-reduction rolls disposed in the end of a continuous casting machine. Japanese Laid-open Patent Publication No. H4-37456 (Patent Literature 5) discloses the art of continuously disposing two or three stages of reduction rolls of twice to five times as much as the thickness of a slab in diameter, and setting the reduction rate of the reduction rolls at the first stage in 1.5 to 4.0% and the reduction rate of the reduction rolls at each second and third stage in 2.0 to 4.5%.